We just moved our core from Cisco to Extreme last month, so I am still a newbie but I can tell you that yes we are doing load sharing and mlags on key servers/equipment connected to our two Extreme X690 cores.
Here is a snippit of some of our ports.  I think it has to be applied in a certain order, I forget but anyway if it doesn't take the sharing command after its an mlag, then undo the mlag and do the sharing command and then do the mlag.... basically you can figure it out.
Same on both switches:
enable sharing 1:1 grouping 1:1 algorithm address-based L3_L4 lacp
enable sharing 1:2 grouping 1:2 algorithm address-based L3_L4 lacp
enable sharing 1:3 grouping 1:3 algorithm address-based L3_L4 lacp
enable sharing 2:7 grouping 2:7-8 algorithm address-based L3_L4 lacp
enable sharing 2:29 grouping 2:29 algorithm address-based L3_L4 lacp
enable sharing 2:30 grouping 2:30 algorithm address-based L3_L4 lacp
enable sharing 2:6 grouping 2:6 algorithm address-based L3_L4 lacp
enable sharing 2:1 grouping 2:1-2 algorithm address-based L2
enable sharing 2:48 grouping 2:48 algorithm address-based L3_L4 lacp
enable sharing 2:9 grouping 2:9 algorithm address-based L3_L4 lacp
On our one X690 stack (we call yellow since its all yellow cables connected to it)
create mlag peer "PURPLE" 
configure mlag peer "PURPLE" ipaddress 1.2.1.1 vr VR-Default 
enable mlag port 1:1 peer "PURPLE" id 101 
enable mlag port 1:2 peer "PURPLE" id 102 
enable mlag port 1:3 peer "PURPLE" id 103 
enable mlag port 2:1 peer "PURPLE" id 201 
enable mlag port 2:6 peer "PURPLE" id 206 
enable mlag port 2:7 peer "PURPLE" id 207 
enable mlag port 2:9 peer "PURPLE" id 209 
enable mlag port 2:29 peer "PURPLE" id 229 
enable mlag port 2:30 peer "PURPLE" id 230 
enable mlag port 2:48 peer "PURPLE" id 248
On our other X690 stack (we call purple since its all purple cables connected to it)
create mlag peer "YELLOW" 
configure mlag peer "YELLOW" ipaddress 1.2.1.2 vr VR-Default 
enable mlag port 1:1 peer "YELLOW" id 101 
enable mlag port 1:2 peer "YELLOW" id 102 
enable mlag port 1:3 peer "YELLOW" id 103 
enable mlag port 2:1 peer "YELLOW" id 201 
enable mlag port 2:6 peer "YELLOW" id 206 
enable mlag port 2:7 peer "YELLOW" id 207 
enable mlag port 2:9 peer "YELLOW" id 209 
enable mlag port 2:29 peer "YELLOW" id 229 
enable mlag port 2:30 peer "YELLOW" id 230 
enable mlag port 2:48 peer "YELLOW" id 248